The rate of DNA synthesis in cultures of chick embryo cells is proportional to the concentration of serum added. The concentration of serum required to stimulate DNA synthesis increases with cell population density and with the duration of culture after trypsinization. The increase of the serum requirement with population density is not caused by the depletion of serum constituents. The requirement of cells for external zinc in DNA synthesis also increases with population density and duration of culture. The kinetics of inhibition of DNA synthesis by deprivation of serum or zinc are similar. Serum deprivation, however, inhibits 2-deoxyglucose uptake and cell movement, but zinc deprivation does not. The deprivation of either serum or zinc inhibits RNA synthesis about twofold. Very low concentrations of actinomycin D prevent the resumption of RNA and DNA synthesis upon restoration of serum or zinc to deprived cultures.

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